Zeolite crystal structures have found a wide range of applications within refinery processes and other processes for manipulating petroleum streams. Some zeolite applications are catalytic in nature, while other applications focus on the ability of zeolites to selectively adsorb molecules within a gas stream.
One type of zeolite structure used for catalytic processing of petroleum streams is ZSM-48, which is a zeolite of the MRE framework type. As for many zeolites, the composition of the synthesis mixture used to form ZSM-48 (or other MRE framework type zeolites) can have a strong impact on the crystalline structure and/or morphology of the resulting zeolites.
U.S. Pat. No. 6,923,949 describes methods for forming ZSM-48 crystals using synthesis mixtures that include non-ZSM-48 seed crystals. The resulting ZSM-48 crystals can have an X-ray diffraction pattern corresponding to ZSM-48, although the presence of the non-ZSM-48 seed crystals can be detected based on activity tests. For crystals having a silica to alumina ratio of about 70 to 1 to 150 to 1, the ZSM-48 crystals formed using non-ZSM-48 seeds are described as being small, irregularly shaped crystals that are free from fibrous morphology. For crystals with a silica to alumina ratio of less than 70 to 1, the ZSM-48 crystals are described as being mixtures of small, irregularly shaped crystals and needle morphology crystals.
U.S. Pat. No. 7,482,300 describes methods for forming ZSM-48 crystals without the use of non-ZSM-48 seeds in the synthesis mixture for forming the crystals. The structure directing agent used for forming the ZSM-48 crystals is described as a hexamethonium salt, such as hexamethonium chloride. The resulting crystals can have a silica to alumina ratio from about 70 to 1 to about 110 to 1, and are described as being substantially free of fibrous morphology. Preferred ranges are also described for the molar ratio of OH− to SiO2 and the molar ratio of structure directing agent (or template) to SiO2. The preferred ranges are described as suitable for formation of crystals that are substantially free of needle-like morphology crystals.
U.S. Pat. No. 8,003,074 describes methods for forming ZSM-48 crystals using a diquaternary ammonium salt structure directing agent with a 5 carbon atom alkyl chain between the ammonium ions (a “diquat-5” structure directing agent). Synthesis of ZSM-48 crystals using mixtures of a “diquat-5” structure directing agent and other structure directing agents, such as a “diquat-6” structure directing agent, is also described. Various types of synthesis mixtures are described that result in formation of fibrous and/or needle-like crystal morphologies.